1. Field of the Invention
The present invention relates to a booster circuit, and more particularly to a booster circuit for use in an LCD (liquid crystal display) controller driver and a driving method thereof.
2. Description of the Related Art
An LCD controller driver is constituted by: a low-voltage circuit section made up of an interface circuit with a CPU, a display control circuit, a memory circuit and the like for storing a content of display; and a high-withstand-voltage circuit section made up of a voltage generating circuit, and LCD driving circuit and others.
The LCD controller driver is used in the LCD unit in a battery-driven portable device such as a cellular phone or a pocket beeper. Therefore, the LCD controller driver contains a voltage generating circuit so as to operate with a single power source. For example, the conventional LCD controller driver contains a charge pump circuit using an external capacity and generates an input power source voltage of 3 V to 9 V (three-fold boosting) or a booster voltage of 12 V (four-fold boosting) to be sued as a power source for the LCD driving circuit.
In a device such as a cellular phone, two lithium-ion batteries (1.8 V) are connected in series to supply 3.6 V to the device. Although the voltage of the battery is lowered with the lapse of operating time, since reduction in power supply voltage of the LCD controller driver affects the display of the LCD, the 3 V voltage is supplied to the LCD controller driver through a constant voltage circuit (regulator circuit) so as not to be influenced by reduction in voltage. If the accuracy of the constant-voltage circuit is .+-.10%, the LCD controller driver must operate with the voltage in a range of 2.7 V to 3.3 V.
The long operation with the battery is required as the market need, and the low-consumption power and the low voltage have been advance. For example, there is such a tendency as that the operating time of the battery is substantially prolonged by using two batteries of 1.2 V to reduce the voltage to 2.4 V. On the other hand, a voltage for driving the LCD depends on the property of the LCD panel, and 6 to 11 V is required irrespective of the voltage of the battery.
In regard to such a demand, although a threshold voltage is lowered with the refinement of the process and the low-voltage circuit section can operate with the low voltage, the voltage of 6 to 11 V is usually required as a voltage for driving the LCD and the withstand voltage of 14 to 15 V must be assured for the high-withstand-voltage transistor constituting the high-withstand-voltage circuit section, which results in difficulty of reduction in the threshold voltage together with the refinement of the high-withstand-voltage transistor.
Therefore, the booster circuit associated with reduction in voltage of the battery is required. The booster circuit is constituted by a charge pump circuit and a control circuit which generates a clock supplied to a gate of a MOS transistor forming the charge pump circuit and controls the operation of the charge pump circuit. Such a booster circuit is provided in the high-withstand-voltage circuit section, and supply of power is necessary for operating the control circuit for the booster circuit. Although the control circuit usually operates with a high voltage generated by the charge pump circuit as a power source voltage, there is no power source voltage supplying means to the control circuit when turning on the power supply. Accordingly, the control circuit does not normally operate at this rate, and a clock can not be supplied to the charge pump circuit, which does not activate the booster circuit. As a countermeasure, in the prior art, a voltage is supplied from an input power source (VDD) line to a high-voltage power source (VLCD) line through a diode in the charge pump circuit as power source voltage supplying means to the control circuit so that this voltage is supplied to the control circuit as a power source.
When turning on the power supply of the booster circuit (initial operation), the voltage is supplied from the input power source (VDD) line to the high-voltage power source (VLCD) through the diode, and the high-voltage power source (VLCD) line receives a voltage (VDD-VF) which is lower than the input voltage (VDD) by a forward voltage (VF) of the diode in the prior art booster circuit using the supplied voltage as the power source for the control circuit. However, when the input power source voltage is lowered by reduction in voltage of the battery, the sufficient high-voltage power source for operating the control circuit can not be supplied by this method.
In the conventional booster circuit, although VLCD in the initial operation is 2.3 V (VDD-VF=3 V-0.7 V) when VDD is 3 V, the threshold voltage of the high-withstand-voltage transistor is approximately 0.7 to 1 V and the operation can be satisfactorily enabled with 2.3 V. However, if the input power source voltage is lowered when VDD=2 V (1.8 to 2.2 V when the power source accuracy is .+-.10%), VLCD becomes 1.3 V and the high-withstand-voltage transistor can not be normally operated. Thus, the high-withstand-voltage transistor constituting an inverter of the control circuit and others can not operate, a clock generated by an oscillating circuit can not be propagated to the gate of the MOS transistor for the charge pump circuit.
As described above, since the necessary high-voltage power source can not be supplied to the control circuit in the prior art booster circuit when the input power source voltage (VDD) is lowered, the booster circuit can not be activated.